Superconducting magnets conduct electricity without resistance as long as magnets are maintained at a suitably low temperature, which is referred to as “superconducting temperature” herein after. Accordingly, cryogenic systems are used to ensure that the superconducting magnets work below the critical [“transition”] temperature of the superconductor.
One conventional cryogenic system uses a cooling coldhead/cryocooler which is mounted to the superconducting magnet. Such mounting of the cooling coldhead to the superconducting magnet has several disadvantages including the detrimental effects of stray magnetic fields on the coldhead motor, vibration transmission from the coldhead to the superconducting magnet, and temperature gradients along the thermal connections between the coldhead and the superconducting magnet. An example of this system would be a “conduction-cooled” system.
Another conventional cryogenic system uses a large volume of liquid cryogen in a cryogen bath to maintain the operating temperature of the superconducting magnet. This cryogen bath (helium can) is exposed to heat loads, such as thermal radiation and conduction from room temperature to the temperature of the cryogen. The liquid helium bath therefore boils off liquid. In some cryogenic systems housing a magnet, the boil off cryogen gas vents to the atmosphere and periodic cryogenic service to refill the cryogen is required. Other cryogenic systems use a refrigerator or a cryocooler to re-condense the boil-off cryogen gas back to liquid. When the refrigeration is turned off, however, by loss of electric power or during periodic system maintenance, or when the magnet quenches and the stored energy of the magnet is dumped into the liquid helium bath, a large amount of boil-off cryogen gas vents to the atmosphere and is lost. Cryogenic service and cryogen refilling are also needed once the refrigeration is turned back on.
One such cryogenic system is routinely used in high resolution NMR spectroscopy to determine molecular structure. For instance, dynamic nuclear polarization is used to enhance nuclear polarization of samples for use in applications such as nuclear magnetic resonance (NMR) analysis including nuclear magnetic resonance imaging (MRI) and analytical high resolution NMR spectroscopy (HRS). MRI has become a particularly attractive diagnostic tool as it is non-invasive and does not involve exposing the patient under the study to potentially harmful radiation such as X-rays.
MRI and NMR spectroscopy, however, lack sensitivity due to the normally very low polarization of the nuclear spins of the materials used. Thus, the dynamic nuclear polarization technique has been developed to improve the polarization of nuclear spins. During the process, a liquid sample is mixed with a polarizing agent and placed in a sample cup which is mounted to a sample holding tube. The sample holding tube is then inserted into the bore of a superconducting magnet located in a cryostat so as to bring the sample to a working volume within the bore, the working volume being located in a microwave cavity defined by a polarization insert. The superconducting magnet generates a magnetic field of suitable strength and homogeneity in the working volume. The sample is cooled and solidified by exposing it to liquid helium (He) in the bore and then irradiated with microwaves while it is exposed to the magnetic field and in its frozen state. The sample is then lifted out of the liquid helium to a position in which it is still subject to the magnetic field, but less homogeneous. Hot solvent is then supplied into the sample holding tube, typically through a dissolution tube/stick or other solvent conveying system, to the working volume so as to dissolve the polarized sample. Thawing in about 10 seconds or less can retain about 50% hyperpolarization in the liquid state. Alternatively, the sample may be melted. The solution or melt is then rapidly extracted and transferred for subsequent use either for analysis in an NMR system or, in the case of in vivo applications, injection into a patient.
One of the drawbacks of the system is the need to move the sample out of the working volume in order to remove it from the influence of the liquid helium prior to supplying hot solvent. This is mechanically complex and costly. Further, problems arise when the helium level in the variable temperature insert (VTI) falls significantly when a sample is loaded and when the dissolution takes place due to the heat dumped into the VTI (causing high He consumption and slow recovery of He level). In addition, the sample holder and dissolution stick are moving parts, expensive to automate. Attempts to solve this problem have utilized a waveguide without contact to liquid He, the waveguide thermally anchored to the thermal shield bore. While the walls are at the temperature of the thermal shield and the base at liquid helium temperature, the sides and base of the microwave cavity are not galvanically or thermally connected and rather electrically connected by capacitive coupling to attempt to act as a non-resonant cavity to localize and concentrate the microwave power density around the sample. Net loss of liquid He, however, is unavoidable.
A hyperpolarizer, like other NMR or MRI magnets utilizes a bath of liquid helium, typically a container for the cryogen. The container and the corresponding tubes for filling the helium container, including the neck tubing connecting the helium vessel to the vacuum vessel, are designed according to the pressure vessel directives. Such directives include the design of safety features that allow safe operation of the hyperpolarizer in operating modes.
In addition, the current status of the hyperpolarizer is complicated. The hyperpolarizer operates at sub-atmospheric conditions which utilize an external buffer volume fed from a helium gas bottle to protect the magnet from air ingress through any feed-throughs or safety features. This is to ensure that leaking safety features, such as burst disks, safety valves, and feed-throughs in the vent stack consume gaseous helium only and consequently protect the magnet from air ingress. Additional operating modes (e.g. at cool-down and magnet ramp (energizing)) necessitate the support of a cryogenic technician. This is especially inconvenient at the customer site, e.g. an MRI suite, particularly as the technician waits for cryogens to be delivered. Further burden is delay as sometimes the delivery schedule changes due to the unavailability of helium.
In the event of the magnet losing its superconducting state and the stored energy dumped into the helium bath, the volume of gas created from about 40 liters of liquid He, for example, is as great as 28,000 L of gaseous He, a volume that is vented off to reduce pressure from the helium vessel during quench. Various components, including the vent stack to remove pressure from the substantial volume of gas during this event, are based on the liquid inventory of about 40 liters He. As used in the system, the He needs to be safely transferred through the dedicated vent stack and released to atmosphere. Valve chattering during this gas release at temperatures at or around about 10 K can also contribute to greater valve leaks upon closure of the valve. When returning to normal operating conditions, the gas flow from the gas bottle to the vent stack increases. This, in turn, requires monitoring of the gas flow. The valve may also need replacement. In this case, further system downtime is to be expected and maintenance and service cost of the vent stack increases.
It is therefore desirable to have a cryogenic system that comprises a very small volume of cryogen in a hermetically closed system so that no cryogen diffuses in or out of the system and venting to atmosphere is eliminated. Thus, a need exists for a component with high-efficiency heat transfer to the magnet to allow the magnet to maintain its supercounducting state and also facilitates quick cool-down or re-cooling of components in a hyperpolarizer system, thereby reducing system cost and allowing the overall system to operate with higher efficiency.
The new hyperpolarizer design will eliminate the need for large quantities of liquid helium, by far less than the current 40 liters of liquid helium, so as to eliminate release of 700 times that volume in gaseous form. The cooling of the magnet will desirably be further simplified to permit efficient operation by a push-button approach without interference or user/operator error.